Expanded graphite sheets have flexibility and are obtained by subjecting a graphite of highly developed crystal structure, such as natural graphite, kish graphite, pyrolytic graphite or the like to an acid treatment with a strongly oxidizing solution such as concentrated sulfuric acid-nitric acid mixture, concentrated sulfuric acid-potassium permanganate mixture or the like to form a graphite intercalation compound, subjecting the graphite intercalation compound to water washing and then to rapid heating to convert it into a graphite powder which is expanded in the C axis direction of graphite crystal, and subjecting the graphite powder to cold processing.
Above-mentioned expanded graphite sheets are superior in heat resistance, chemical resistance, sealing property, stress relaxation, etc. and are therefore in wide use as a gasket material or an electromagnetic interference shield material for automobile engine, atomic reactor, ship building or general industrial use.
As the performance requirements for the above applications have become higher, the performance requirements for the expanded graphite sheet used therein have become higher as well. Expanded graphite sheets are considered to have problems particularly in mechanical properties (bending and tensile strengths) as well as in leakage and significant strength reduction both caused by the infiltration of liquid thereinto.
In order to solve these problems, use of reinforcing material for expanded graphite sheet was proposed. As the reinforcing material, there are used a metal plate, a metal foil, a metal net, a high-strength fiber and a fabric made of said fiber; and gaskets, etc. are in actual production by attaching such a reinforcing material to an expanded graphite sheet with an adhesive. As this adhesive, there have been mainly used those of rubber type, epoxy resin type, acrylic resin type, phenolic resin type or polyamide type. As the performance requirements for expanded graphite sheet have become higher as mentioned above, however, it has come to be pointed out that the above conventional adhesives are not sufficient in heat resistance, adhesive strength, chemical resistance, etc.
Meanwhile, adhesives of polyimide type or polybenzimidazole type with excellent heat resistance have been developed in recent years. These adhesives, however, have problems in that they contain a particular high-boiling solvent, must satisfy severe (high temperature and high pressure) adhesion conditions owing to the physical properties, are expensive, and are used in limited applications.
In order to solve these problems, modification of epoxy resin was attempted. For example, Japanese Patent Application Kokai (Laid-Open) No. 260669/1985 proposes a heat-resistant epoxy resin type adhesive, for example, comprising polyfunctional epoxy resin as triglycidyl ether of triphenol, an aromatic diamine as curing agent and an inorganic filler. Also, Japanese Patent Application Kokai (Laid-Open) Nos. 89380/1984 and 79079/1985 propose epoxy resin type adhesive compositions each comprising an epoxy resin and a rubber component dispersed therein.
These conventional compositions, however, are solid and have a short pot life, and are inferior in workability and insufficient in heat resistance. Further, Japanese Patent Application Kokai (Laid-Open) No. 30122/1987 describes a composition obtained by adding a thermosetting bismaleimide compound to a polyimide resin to achieve improved thermal stability of adhesive strength. This composition, however, has a high curing temperature of 275.degree. C. and is far from practical applicability in production of gasket, etc.
The present invention is intended to provide a heat-resistant adhesive composition which can be used in adhesion of flexible material such as expanded graphite, etc. under mild conditions and which is superior in heat resistance, chemical resistance, and adhesive strength particularly at high temperatures.